The "video history" of the crashed USAF ship makes it very clear that the planet is uninhabited when they "landed". I can understand how a race of apes develops - they had a bunch of them on board. I can understand how a race of humans develops - they are descendants of the original crew. What I don't understand is...where the heck did all the horses come from?
Linda Harrison, who portrayed Nova in the original Planet of the Apes and its sequel Beneath the Planet of the Apes, has a cameo appearance in this movie as one of the people inside the rolling cage as it's being taken into the city. She is seen standing next to Mark Wahlberg, shaking her head when he asks her a question.